Characterization Of Structure And Properties Of Ethylene-tetrafluoroethylene Copolymers

Ethylene-tetrafluoroethylene copolymer (ETFE) is one kind of high-performance engineering plastics, with excellent heat and chemical resistance as fluoropolymer, and melt-processable characteristics as polyethylene. Due to the limited studies on the synthesis, structure control technology and relationship between structure and properties of ETFE in our country, the production and application of ETFE are still developing. To aid the development of ETFE, the structure and properties of ETFE was studied in this thesis. The characterization methods for ETFE composition, mean molecular weight (MW) and molecular weight distribution (MWD) were established, and thermal transition, crystallization behavior and Theological properties of commercial and pilot produced ETFE were investigated.The compostion of ETFE was determined by oxygen flask combustion-ion chromatography method. The dynamic Theological analysis method was modified to measure the MW and MWD of ETFE, the zero shear viscosity was obtained by using Carreau-Yasuda equation, which was further used to calculate the weigh average MW of ETFE. The relationship between MW and meltindex (MI) was estabilished as follows: MI=2.16×10¹²M_w^-1.93. According to the general Maxwell model, the calculation of plateau modulus (G_N⁰ ) and K₂ was proposed. Thus,MWD of ETFE could be calculated from the dynamic modulus frequency relationship.According to XRD results, it was found that ETFE exhibited the orthorhombic, pseudo-hexagonal and hexagonal lattic, when the molar composition of TFE was less than 50%, between 50～75% and greater than 75%, respectively. With the increase of TFE molar fraction in ETFE, the crystallinity of ETFE had little change, and the crystal size was slightly decreased.The crystalline melting behavior of ETFE resin as-polymerized and ETFE as-recrystalized was characterized by different scanning calorimetry (DSC). It was found that the melting temperature (T_m) of ETFE resin as-polymerized was increased with the increase of TFE molar fraction (f_TFE) initially, and reached a maximum at about 53%. Then, T_m was decreased as f_TFE increased, and reached a minmum value at 65%. T_m was increased with the increase of f_TFE again when f_TFE was greater than 65%. The melting enthalpy of ETFE was increased with the increase of f_TFE and then decreased when f_TFE was greater than 65%. T_m and melting enthalpy of ETFE as-recrystallized were increased as the heating rate increased due to the over-heat and heat disequilibrium. T_m of ETFE as-recrystallized changed no more as the cooling rate varied, while the melting enthalpy of ETFE was decreased as the cooling rate increased. ETFE thermal treated with different annealing temperature and time showed three peak temperatures in DSC curve.Nonisothermal crystallization kinetics of ETFE was studied by DSC. The results indicated that the maximum crystallization temperature of ETFE was decreased, and the range of crystallization temperature became broader as the cooling rate incresed. According to results processed by Avrami equation and observation of polarizing microscope, the crystallization process could be divided into three phases, named nucleation period, primary crystallization phase in which nucleus grow into spherocrystals, secondary crystallization phase in which spherocrystals collide mutually and grow continually, respectively. Mo equation (lgφ= lgF(T)-a×lgt) was also applied to fit the nonisothermal crystallization kinetics of ETFE. It was showed that with the increase of crystallinity, a value calculated almostly kept constant, and F(t) value increased. The activation energy of nonisothermal crystallization of ETFE was increased with the increae of f_TFE.Studies on rheological properties of ETFE showed that the melt shear stress was gradually increased, and the apparent viscosity was gradually decreased as the shear rate increasesd, indicating the shear-thinning phenomenon of ETFE melt. ETFE melt was close to Newtonian fluid as the temperature increased. The flow activation energy of ETFE melt decreased with the increase of the shear rate.In the aspect of mechanical properties of ETFE as-recrystallized, it was found that the elongation at break, Young's modulus and bending modulus were increased, the tensile strenght, bending strength and impact strength were increased first and then decreased, as the thermal treatment temperature increased. Keeping the annealing temperature at 230℃, the elongation at break, bending modulus, bending strength and impact strength were increased, the tensile strengths and Young's modulus were decreased with the increase of annealing time.The above results would guide the synthesis and processing of ETFE.